Development of alumina forming ods ferritic superalloys as new biomaterials for surgical implant (ALUSI)

The Fe-20Cr-5Al alloy (PM 2000), coated with alumina by thermal oxidation, could replace current biomaterials used for the fabrication of surgical implants. General advantages with respect to current biomaterials are: - Absence of Ni (allergenic and carcinogenic element) and Co (geostrategic element). - Possibility of developing an outer alumina layer of about 5 microns by thermal oxidation. - A reduction of about 20% in density with respect to that for Co-Cr alloys. - Mechanical properties of the final product are equal or superior to those of biomaterials in use. - Superior corrosion resistance and very low ion release, even under friction conditions, especially in the preoxidised condition. - Good biocompatibility, often approaching the behaviour of materials well known for their biological acceptance, e.g. alumina - Wear properties could approach that of bulk alumina, assuming that a final mechanical polishing yields the roughness required at the ISO standard for ceramics (20 nm). General advantages with respect to other biomaterials coated with bioinert coatings are: - High level of compressive residual stresses at the coating without compromises the scale adherence. This implies a high fatigue limit (about 550 Mpa) and a high level of deformation of the coating without cracking (about 1%). General advantages with respect to other surface modification techniques are: - Thermal oxidation is a low cost and non-contaminant process, easy to perform. - Coating is homogenous in thickness irrespective the shape of the component. - Dimensional tolerance of the component does not change during preoxidation. General advantages with respect to bulk alumina are: - High versatility when designing (shape, wall thickness) the components. - The alloy is easier to machine due to the less hard nature, which results in an increase in service life of tooling and no need to use expensive tools. - Components can be elaborated by the producer of current metallic implants with their own facilities. The only technical disadvantage, that however could be useful for certain medical devices, is that the alloy exhibit a soft ferromagnetic behaviour which obviously prohibits the use of medical control techniques based on strong magnetic fields (NMR). Therefore, alternative control techniques should be used.

Surgical implants of a Fe-Al-Cr intermetallic alloy, with Al contents in the range of 30-40 at%, Cr contents in the range of 8 to 12 at%, and yttria additions, could replace current biomaterials. A powder metallurgy route was settled during the project. General advantages with respect to current biomaterials are: - Absence of Ni (allergenic and carcinogenic element) or Co (geostrategic element). - A reduction of about 38% in density with respect to that for Co-Cr alloys. - Tensile strength higher than that for current metallic biomaterials. - Corrosion properties similar to that of current biomaterials. - Similar polyethylene wears rates than that for current metallic alloys. - Good biocompatibility in vitro, often approaching the behaviour of materials well known for their biological acceptance, e.g. alumina. - Possibility of generates an outer alfa-alumina scale by thermal oxidation, which could enhance the surface related properties (corrosion, wear).

Industrial pilot scale production by Powder Metallurgy of non-ferromagnetic Fe-Al-Cr alloys was completed. Selected alloys have high yield strength (up to 1300 MPa) and some of them reasonable ductility (up to 9%). All they show a good corrosion resistance in vitro that is similar to that of the current biomaterials. None of the alloys developed during the project was found to be cytotoxic, even when tested, as fine particles were not causing injury to Saos-2 and J774A.1 cells. The uptake of such particles by cells following phagocytosis, with no induction of cell death, was an additional proof of the lack of toxicity of the alloys, in contrast with other authors who found that small ingested particles are most cytotoxic to cells. When solid surfaces have been assayed, all the candidate alloys have been shown to have good biocompatibility, often approaching the behaviour of materials well known for their biological acceptance, e.g. alumina. The finding of human osteoblasts adhering, spreading and eventually covering with multilayers the surface of the alloys, is strengthening the result of good compatibility in vitro. The data base of FeAlCr intermetallic alloys processed by Powder Metallurgy including mechanical, chemical and biological properties would allow the identification of potential applications for the fabrication of surgical implants and medical devices.

Preoxidation of PM 2000 leads to the formation of an outer a-alumina scale fine, dense and tightly adherent to the substrate. Surface roughness increases about one order of magnitude as a consequence of the formation of small oxide nodules, the density and size of which depend on the temperature of oxidation, exposure time and surface finishing. In general the number and size of the nodules increased with increasing temperature and/or exposure, i.e. scale thickness. Possible procedures to suppress nodule formation during oxidation have been analysed. The oxide nodules formed during the first hour of exposure depend strongly on the temperature and the oxygen partial pressures. The samples exposed in vacuum during the first 90 sec formed a pure alumina layer, whereas the samples that were introduced into the air-filled furnace showed an iron-chromium-oxide mixed surface layer underneath that the alumina formed. Mechanical polishing of the alumina-coated material may reduce the roughnes to about 30-40 nm. Further effort should be addressed to reduce this value to 20 nm that is required for bulk ceramics used for wearing parts

After a thermal oxidation (1100ºC) the alloy PM 2000 is coated in situ with an alfa-alumina layer. Important to remark that thermal oxidation, applied in the last stage, and consequent alumina scale formation, did not affect the dimensional tolerance of the investigated components. The alumina-coated alloy presents high tensile strength (930 MPa), high elongation (24%), and elevated impact strength (250 J/cm2). In addition, the candidate alloy exhibits sufficient bending fatigue strength (fatigue limit of about 550MPa). Interesting to note that the elevated compressive residual stresses at the coating accounts for the capability of the alloy to be deformed up to about 1% without compromising the scale integrity, which is a great advantage when considering the 0.1-0.2 % elongation for bulk alumina. Long-term wear tests in a join simulator reveal that the UHMWPE polyethylene wear of the uncoated alloy is similar to that of CoCr alloy. The worst behaviour of the alumina coated material with respect to the bulk alumina results from the higher roughness of the coating. Further investigation is needed to reduce the roughness of the alumina coated alloy to the value of 20 nm indicated in the ISO standard. The alumina layer is fine (a few microns) but dense and tightly adherent to the substrate, which accounts for a superior in vitro corrosion resistance. Therefore, a very low ion release is expected to occur in vivo. Cell culture tests, including primary human osteoblasts reveals an excellent biological acceptance. It has been found that osteoblasts are able to produce ALP, a specific marker of cells with bone-forming activity. In this respect, PM 2000 holds the promise to be suitable substrate for bone integration. A data base of the PM 2000 with and without scale collecting mechanical, chemical and biological behaviour at ambient temperature would allow the identification of potential applications for medical devices.

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